This invention relates to laser systems and more particularly to laser systems that use the properties of incoherent light to produce a smooth and controllable spatial illumination profile at the focus of an output lens.
Direct-drive laser fusion requires concentrated laser light that uniformly illuminates a spherical target surface. Theoretical studies have shown that acceptable spherical uniformity can be achieved with direct illumination by overlapping a limited number (more than or equal to 20) of focused beams, provided that each individual beam profile is smooth and reproducible. Earlier efforts to obtain such profiles had been frustrated by the inherent imperfections in high power multistage lasers. The cumulative effect of small amplitude and phase aberrations introduced by each optical element of a multistage laser is to produce large random aberrations in the output beam. In the usual configuration, where the target is placed in the quasi near-field of the focusing lens (between the lens and its focal point), these aberrations tend to produce large random intensity nonuniformities at the target surface.
One possible solution to this problem, which is described in U.S. Pat. No. 4,521,075 to Stephen P. Obenschain and Robert H. Lehmberg, is a technique called Induced Spatial Incoherence (ISI). In this scheme, spatially incoherent light is created by propagating a laser beam of broad spectral bandwidth .DELTA..nu. (delta nu) through a pair of echelon structures that impose a different time delay at each step. If the delay increments .DELTA.t are chosen somewhat larger than the optical coherence time t.sub.c =1/.DELTA..nu., the beam is sliced into an array of mutually-incoherent square beamlets. Each of these will independently focus to the same diffraction profile (the so-called sinc.sup.2 function) of width f.lambda./d, provided that its initial width d is small in comparison to the transverse scalelength s.sub.a of the incident beam aberration. (Here, f is the focal length of the lens, and .lambda. is the mean optical wavelength.) The transient interference pattern produced by superposition of these beamlets will evolve randomly in times of order t.sub.c. The target will therefore ignore this rapidly shifting structure if its hydrodynamic response time t.sub.h satisfies t.sub.h &gt;&gt;t.sub.c. For example, an optical bandwidth .DELTA..upsilon.=30 cm.sup.-1 (easily achieved in Nd:glass or KrF lasers) provides t.sub.c =1 psec, whereas t.sub.h is typically about 1 nsec.
In its present application, the ISI technique requires the echelons to be placed at the output of the laser. This restriction stems from self-focusing effects in glass lasers, and from the necessity of maintaining spatial coherence in the harmonic conversion crystals (used to generate the short wavelengths needed for fusion-related experiments). The beamlets would quickly and catastrophically self-focus if one attempted to amplify them in a multistage glass laser, where the nonlinear phase shifts are typically about 5-10 radians. By placing the echelons at the output, one can control self-focusing and thus maintain an acceptable degree of transverse beam uniformity over distances comparable to the beamlet width d. Unfortunately, this configuration would require an excessive number of large and expensive AR-coated steps operating at high optical fluence levels if it were used in a fusion reactor with large apertures and multiple beam lines. For example, a recent conceptual design study for the Sirius-M reactor concluded that in order to use ISI at .lambda.=1/4 .mu.m, one would require 240 steps in each transverse direction for each of 32 drive beams. Another issue raised by this ISI configuration is that of efficiency. Approximately 18% of the energy at the focal plane will diffract into sidelobes, and most of that energy would have to be discarded in order to achieve good illumination uniformity.
Although self-focusing remains a serious problem in glass lasers, it will be far less important in angularly-multiplexed KrF systems, where the amplifying medium is gaseous, and the optical intensities are typically much lower. A significant reduction of the self-focusing problem (e.g., reduction of the nonlinear phase shift to less than 1 radian) could eliminate the necessity of placing the echelons at the laser output, thereby opening the way for several possible improvements. For example, instead of the reactor configuration discussed above, one could produce the beamlets by a single pair of echelons at a low power stage, spatially-filter them to eliminate the sidelobes, then optically-relay them through each of the 32 main amplifier chains. As long as these beamlets remain small in comparison to the transverse scalelength of the aberrations (e.g., due to passive optics, turbulence, and nonuniformities in the amplifier excitation), they can focus to the sinc.sup.2 profile without any sidelobes at the target.